I read somewhere that Quad performs a heat treatment after tensioning the diaphragm to 'set' the film.
I am curious if anybody has a clue or good guess as to what perpose this serves. I mean what are the effects on the performance of a panel? Does the tension hold longer? I'm also wondering how one would deduce the obvious parameters like temperature and duration of the heating.

the heat treatment is very commonly used with ESLs.
First : it is very easy to do with a heatgun e.g. and without the need to build a extra tensioning frame
second: the mechanical tension is relatively low, but within small tolerances. So its good for serial production (and You need a series of two at least ;-))
third: the tempering releases internal material stress. The panel works like ´played´ (broken in(?)). So the usual 50-100hours break-in time -where parameters and sound change a bit- reduces considerably.
fourth: If the membrane looses tension over time it can be retensioned by heating again -which is impossible with mechanically tensioned membranes.

The important parameter is temperature. As soon as You reach the needed degrees (~150°C) the material shrinks. It won´t shrink further by longer heating or hgher temperature, but it dosen´t harm to go over the membrane twice either ;-)

thanks for the reply Calvin, but I think we might not be talking about the same thing here. Perhaps I was not very clear stating my question, because I don't mean heat shrinking. I am talking about an after-tensioning heat treatement, maybe (probably?) at modest temperatures for longer time. At least that is the process as I understand it.
But perhaps a number of the advantages that you talk about are also a factor here, I don't know. Thanks anyway.

Treating the membrane with lower temperatures (a hairfohn instead of a heatgun (paint remover), is said to decrease differences in membrane tension. I've tried this once without audible results, don't know if this is true.

I believe that Quad diaphragms have always been Mylar. Mylar is a polyester film, and most grades don't heatshrink much at all: 1 - 3%, just before the melting point. Also the amount of shrinkage varies with direction.

Mylar is cast on very large machines, and the master roll is sliced into still large, but managable rolls. These tend to be consistent through the roll. I also find it astonishing how well the manufacturers can control parameters from one batch run to another.

As the yield is huge on each roll (I have one next to me now with 2,200 metres of film on it), most panel manufacturers find one roll lasts quite a long time, so variation between rolls isn't much of a problem!

Quad used to use elaborate stretching jigs to mechanically tension the film. I have seen a photo of one, but I can't remember where. I don't know if they still do this - it was an old photo.

However, the dust covers (material unknown to me) that are applied over the stators may well be heatshrunk. I certainly believe that a heat gun is a standard way of re-tensioning them if they develop wrinkles over time.

Some 20 years ago I ordered replacement tweeter panels from the factory for my Quad ESLs (the ESL-57, although The Acoustical Manufacturing Co. Ltd. themselves never used this type number). They included a mounting instruction in the kit, and the dust cover had to be tensioned with a hair dryer after having mounted the unit in place. Important was the continuous movement of the hair dryer so that no hole be burned. I applied this method also for the bass panels. There I used polypropylene foil that needed more heat in order to shrink. The goal of the whole process is to tension the dust cover up to the point where the wrinkles disappear and it won't rattle any more. You don't have to do any heat treatment on the diaphragm itself.

This photo was supplied by Quad in England a long time ago, while I have seen the machinery, including the "Mystery Box" at the end of the stretching jig, with it's door open, at the German http://www.quad-musik.com/html/english.html site, but can't find it there now...

The "Mystery Box" has never been described on any of the Quad Refurb sites - Stokes, Jacobsen, or King - and discussions of tension are pretty thin as well...

Originally posted by torroid88 ... while I have seen the machinery, including the "Mystery Box" at the end of the stretching jig...
The "Mystery Box" has never been described on any of the Quad Refurb sites - Stokes, Jacobsen, or King - and discussions of tension are pretty thin as well...

I suspect that it is indeed an oven. My guess is that the process must have been something like this: The film is mounted to the stator on one end of the line and is laying on some sort of sled that is then moved into the oven. The second sled is larger so one can move over the other. While one is in the oven the other is created. At least, that is how it looks to me.
What is not clear to me is if the heat setting is/was done only for the 63 or also for the original ESL..

The oven looks like the type used on package shrinking assembly lines or pizza heaters.

Thanks for the wiki, perhaps I should ask Dupont about the right temperature and duration. They must have some info on this.